The embalmed heads, tagged as human specimens, were packed in three coolers shipped on a Lufthansa flight just before Christmas, according to a spokesman for the medical examiner's office, the Chicago Tribune reported.

The newspaper reported that the shipment was held up because the final destination was not clearly noted on all the documentation.

Customs then X-rayed the coolers and determined that they contained 18 human heads still covered in skin. One cooler was opened for a preliminary inspection, the medical examiner's spokesman said, according to the Tribune.

The Chicago Sun-Times quoted Brian Bell, a Chicago-based spokesman for the Department of Homeland Security, as saying, "We need to make sure that they are truly used for medical research purposes."

The Tribune reported that a crematory in Schiller Park, Ill., came forward with what appeared to be the proper documents after news of the unclaimed heads surfaced.

ABC News reported that researchers in Rome had been using the embalmed heads but shipped them to the United States for cremation at the facility near Chicago.

"The crematory has done nothing wrong. In fact, they were just here to hand all the paperwork over, and we're satisfied everything is on the up-and-up," Tony Brucci, chief of investigations at the Cook County medical examiner's office told ABC News.